Today, most digital presses rely on an optical sensor for calibrating colors, and since different types of sensors measure different types of data, the type of the optical sensor is usually linked to the method of the color calibration.
There are two commonly used methods for measuring, and hence calibrating, color; the first method measures optical density (OD) values of a printed area and the second method measures a full spectral response of a printed area and therefore its CIE 1976 (L*, a*, b*) color space (also called CIELAB, whose coordinates are L*, a*, and b*) colorimetric coordinates.
Obtaining accurate color measurements typically depends on the precise positioning of the sensor over the printed area. Inaccurate position may result in the sensor not measuring accurately a consistent patch and may therefore lead to inaccurate or inconsistent measurements. Obviously, the accuracy requirements increase as the printed patches decrease in size. Since the press set-up state may degrade over time as consumables age and other process variables change, the resulting color may also alter over time. To prevent and control this color drift, and to increase color consistency both over time within a single press and job and across presses, a Continuous Color Calibration (CCC) process was developed in which a set of calibration patches is printed on each sheet in a job alongside the customer's content.
Reference numerals may be repeated among the drawings to indicate corresponding or analogous elements. Moreover, some of the blocks depicted in the drawings may be combined into a single function.